Process obtaining landfill disposable wasted from hydrocarbon containing sludge

ABSTRACT

A method of processing oily sludge to remove a substantial proportion of the water and hydrocarbon content therefrom to form a solid cake of waste material that is suitable for disposal in land fill sites. The method involving, firstly promoting flocculation in a batch of waste material by mixing in a flocculating agent. The flocculated waste material is then pressing at a first pressure to extract substantially all the free water within the mixture. After the first pressing the waste material is subjected to a second pressing at a second pressure to extract a substantial proportion of hydrocarbon material and further water from the mixture, thus leaving a solid cake of waste material. The invention also considers additional processing steps to maximize the level of hydrocarbon material reclaimed from the waste material.

FIELD OF THE INVENTION

The present invention relates to the field of waste material treatmentand more specifically to the area of the treatment of sludge thatcontains, amongst other things, hydrocarbons.

BACKGROUND OF THE INVENTION

The processing and disposal of waste material is an important area ofenvironmental management. Generally speaking waste material is eitherburnt or buried at landfill sites. Some waste is not suitable fordisposal by burning and is thus buried (landfill) or simply spread overan area of land (landfarm). However, some waste material is not suitablefor such disposal methods.

Oily sludge is an example of a waste material that cannot suitably bedisposed of by burning. Also, because of the fluid nature of oily sludgeit can not be simply buried at landfill sites either. This is because,under present legislation, waste material must be buried in a solidform. The practice of landfarming is also undesirable for environmentalreasons.

The appearance of said sludge is that of thick mud and it is usuallyunsuitable to be further dewatered by pressing (e.g. belt press, chamberpress) or centrifuging to the required dryness and shear stability forlandfill; especially without chemically treating the sludge.

Under impending legislation waste, nominally solid, having an oilcontent of greater than certain specified values (i.e. 0.1%, 5%, 6%)will result in the material being classified as hazardous. The specifiedoil content of a particular batch of waste will determine the eventualroute of disposal available for the waste.

Waste with an oil content of between above 0.1% and 5% can be disposedof at hazardous waste sites and non-hazardous waste sites, provided theyhave special cells. Waste having an oil content of 6% and over must bedisposed of at a hazardous waste site. This distinction can createsignificant cost differences between batches of waste having an oilcontent of 5% and waste having an oil content of 6%, mainly due totransport costs and landfill site costs.

In view of the legal, financial and environmental requirements, it isobviously more preferable to produce waste having an oil content of lessthat 6%, and yet more preferably 0.1% or below. In view of the abovesituation, at present oily sludge is processed to ensure that it is inthe necessary state for disposal.

The pretreatment processes currently in use for oily sludge, whichinclude the solidification of the sludge using absorbents, are bothexpensive and wasteful as they may not involve the reclamation of any ofthe hydrocarbons held within the sludge. Other techniques such asthermal drying and desorption of hydrocarbons may be used but these areexpensive and energy intensive processes.

In light of the state of the art there is a need for a method ofprocessing oily sludge to transform it into a form that can be moresuitably disposed of. The processing method of the present invention istargeted mainly at the treatment of oily sludge that arises fromrefineries and waste oil recovery/treatment plants, where waste oil andoily sludge are recovered to make fuel oil from industrial waste oil andgarage forecourt recycling schemes.

It is generally accepted that plate and frame (or chamber) presses arenot suitable for processing oily sludge due to the likelihood ofblinding. As a result, belt presses are usually used in the processingof oily waste. Typically, a belt press can only achieve a maximum of 20%solids and a minimum of 10% oil, which as a ratio of 2:1. There istherefore a need for a method of achieving waste with a much lower solidcontent to oil content ratio.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of processing oilysludge to remove a substantial proportion of the water and hydrocarboncontent therefrom to form a solid cake of waste material, comprising thesteps of:

a) providing a batch of a waste material;

b) promoting the flocculation of the waste material by mixing in aflocculating agent;

c) firstly pressing the mixture at a first pressure to extractsubstantially all the free water within the mixture; and

d) secondly pressing the mixture at a second pressure to extract asubstantial proportion of hydrocarbon material and further water fromthe mixture, leaving a solid cake of waste material.

Preferably, additional water is added to the waste material before aflocculating agent is added to the waste material. Although the additionof water greatly assists that flocculation of the waste, it is notalways essential with some flocculating agents.

Preferably, the water is added to the oily sludge in a volume/volumeratio of 2:1.

Alternatively, the amount of water added is such that the flocculentused in stage b) of the method is diluted to between 0.1 times and 50times the volume of the waste material.

Once the solid cake of waste material has been produced it can bedisposed of in a landfill site.

The present invention is intended to be utilized to process sludge wastematerial with hydrocarbon content.

Preferably, the first pressing applies a first pressure of up to about 7bar. The second pressing may apply a second pressure of 13 bar or above.

Preferably, the flocculating agent is a synthetic polyelectrolyte of anacrylamide copolymer type. Preferably, flocculation is promoted bymixing in a 0.5% solution of a cationic polyacrylamide flocculentemulsion polymer at a dose rate between 0.1 and 15 kg/tonnes of drysolids. The flocculation of the mixture may be improved by the gentlestirring of the mixture.

It will be appreciated that flocculation may be achieved by the use ofany suitable form of flocculation system including, but not limited toacrylamide copolymers, multiple component systems based on coagulant andflocculent sequential additions, inorganic mixed components such asbentonite and cationic acrylamide and synthetic micro-particle systems.

The filtering of the mixture to extract the free water is carried outusing a chamber press with a suitable filter clothing.

It may also be preferable to use a chamber press to extract thehydrocarbon material and further water from the mixture. Alternatively,a centrifuge can be used to apply the necessary pressure to themixtures.

Preferably, the pressure of above 13 bar is maintained for 2 hours so asto remove the optimum amount of hydrocarbon material from the mixture.

The steps of removing the free water and then the hydrocarbon materialand further water could be carried out using the same chamber press.

A further benefit could be gained if the oily sludge was screened priorto the addition of water, so as to remove large debris from the sludgematerial.

Another benefit could be gained by pre-washing the oily sludge with ahydrocarbon based solvent before the water is added to the sludge, so asto remove more hydrocarbon material. A low boiling point solvent may bereadily removed from the cake to achieve very low residual hydrocarboncontent by evaporation.

The waste material may be washed in situ within the press, at any stageof the process, with a suitable oil dispersant surfactant solution. Thetype of surfactant used will vary depending on the different types ofoil present in the waste material. Using chemically compatibleproprietary surfactant packages at any stage of the process leads to afurther reduction in the oil content of the final solid cake.

It will also be appreciated that the oil content of the final solid cakeitself may be further reduced by a further round of washing in asurfactant solution followed by re-pressing.

Preferably, the water used for this pretreatment could be the free waterextracted from the oily sludge during the processing of an earlierbatch.

DETAILED DESCRIPTION OF THE INVENTION

For the sake of the present invention, the preferred waste materials ofoily sludge should be understood to be sludge that results fromprocesses wherein the resultant sludge is a mixture of liquidhydrocarbon, organic liquids or natural fats, water and solid materialin any proportion. Usually, the maximum percentage values of thecomponents of such sludge are about 40% for liquid hydrocarbon, about90% for water and 20% for solids.

The process on the invention deals with these types of sludge byaddressing the fundamental problem associated with a three phase systempresented in sludge. Typically a sludge is presented that has therheology associated with mud or the like.

Analysis shows the following content of a typical oily sludge:

Hydrocarbon 20% Solids 10% Water 70%

It is known that the addition of flocculents such as polyelectrolytesmay be used to assist in the dewatering of regular solids/water sludge.Normally an admixture of an appropriate synthetic polyelectrolytesolution in water to a slurry will produce flocculation of the solids sothat water may be drained therefrom using a suitable dewatering devicesuch as a filter press or centrifuge. The resultant cake produced willbe solid in nature if enough water is removed.

However, in the case of oily sludge it is not enough to remove just thewater as the resultant material will still contain hydrocarbon fluids,which in admixture with the solids therein produces a furthersludge-like material, albeit at a lower level of water content. Suchmaterial is still semi-solid and cannot be disposed of as landfill.

Further, it was found that for oily sludge, large excess volumes ofwater are required before flocculation can take place. Thus the amountof sludge that would need to be treated would greatly increase in volumeprior to dewatering. Thus in the present invention it is preferable toadd water to the sludge to increase the volume. However the addition ofwater is not essential, as it is the stability of the floc within thepress, during filling and squeezing, which is most important.

The mixture was stirred and admixture of a suitable flocculent,preferably synthetic cationic polyelectrolyte, was added to causecomplete flocculation of the sludge to an oil/solids coagulate and waterexternal phase.

The mixture was then admitted to a chamber press, with suitable filterclothing, at pressures up to about 7 bar. The press filled with sludgeand essentially oil free water was discharged.

The press was opened and an oily sludge, which was of a reduced volumeto the original sludge volume, was found to be produced. However thissludge was still unsuitable for landfill, as it was semisolid and stillfull of oil.

The treated sludge was again admitted to the chamber press, but afterfilling, the inlet valve to the chambers was closed. The membranes inthe inner chamber spaces were inflated to a pressure of above 13 bar(about 15 bar) for approximately 2 hours. A further discharge wasobserved from the press which consisted of a small amount of water alongwith oil, both free and emulsified.

After this procedure, upon opening of the press a solid cake, which wassubstantially reduced in water and oil, was found to have been produced.A sample press produced 71% dry solids, with approximately 5%hydrocarbon content. This cake was suitable for landfill in a hazardouswaste landfill site.

The sludge volume reduction was substantial and the cake consistency wasbrittle.

Experimental

A typical refinery waste sludge was obtained from a UK refinery andtreated as follows.

Sample (200 ml) was diluted 2:1 with fresh water and resultant mixturewas homogenized by stirring. The sample volume now 600 ml.

Addition of 100 ml 0.5% polymer solution of Treatchem 104 supplied byTreatchem Ltd, Whitchurch, England was made with gentle stirring.

The whole mixture flocculated to give a granular appearance and freewater.

The mixture was admitted to a Buchner filter funnel fitted with a wovenfilter cloth of polypropylene and allowed to drain.

Essentially only free water was removed and the water separated was verylow in free oil. The resultant drained sludge had a volume of less thanthe original 200 ml but was essentially sloppy and extremely oily.

The content of the filter was then emptied into a woven piece of clothwhich was folded around the sludge and gently screwed up such thatexcess pressure was applied to the sludge. It was noticed that asubstantial amount of oil and water started to separate through thecloth and that the contents therein were becoming more solid in nature.

To confirm the feasibility of the technique on an industrial scale apilot scale, chamber press was obtained for trials. The chambers were300 mm in diameter and the press was fitted with membrane plates capableof exerting pressures up to 15 Bar.

An 8 kg sample of oily sludge from a waste processing facility was takenand screened to 1 mm to remove large debris. The sample was mixed withwater (12 liters) and conditioning flocculent added as a 0.5% solutionuntil complete flocculation had taken place.

The mixture was admitted to the press via a small air driven piston pumpup to 7 bar when the press was full and excess water had been removed.Again very little oil was seen in the filtrate up to this point Thefilling of the press was rapid (approximately 15 minutes).

The inlet valve was closed to the press and pressure applied to themembranes at 15 bar for approximately 2 hours. A discharge of oily waterwas observed from the press which became richer in oil over time. Thevolume of discharge decreased with time as the contents of the press area fixed amount.

Upon opening the press a 1.8 kg hard cake was discharged containingapproximately 5% oil (toluene extractable) and approximately 71% drysolids. The cake was adjudged suitable for admission to a hazardouslandfill site.

Repeating the same experiment using 7 bar squeeze on the sludge for 2hours produced a sloppy oily sludge. It now becomes apparent that atwo-stage press is required to obtain removal of the hydrocarbon fluid.

The method may be extended to washing the sludge beforehand with asolvent, especially of a low boiling point or volatile nature, to removemore hydrocarbon and the water used may be recovered water from thepress.

The two-stage method is applied in a single device comprising a membranefilter press. The pressure required will be driven by the sludgecharacteristics. It is noted that the pressure required to squeezeorganic material/oily material from the waste material depends upon theviscosity/solvency of the organic material/oily material. In fact, itwas found that white spirit could be exhausted from a sample of wastematerial using the present method to apply a pressure of only 3-4 bar.

Whilst it is discussed above that a pressure of 13 bar or above for thesecond pressing stage is the preferred minimum requirement, thepreferred maximum is only limited by the technical tolerances of theseparating device being used to press the waste. material

It is appreciated that although the process of the present invention isparticularly suitable for the processing of oily sludge, the processcould also be used to process other mixtures to remove the requiredcontents.

1. A method of processing oily sludge waste containing hydrocarbon oilto remove a substantial proportion of the water and hydrocarbon contenttherefrom to form a solid cake of waste material, comprising the stepsof: a) providing a batch of an oily sludge waste material; b) promotingthe flocculation of the waste material by mixing in a flocculatingagent; c) pressing the mixture in a membrane chamber press at a firstpressure to extract substantially all the free water within the mixture;and d) subsequently pressing the mixture in the same chamber press at asecond higher pressure to extract a substantial proportion ofhydrocarbon material and further water from the mixture, leaving a solidcake of waste material, wherein the first pressure is up to about 7 bar,and the second pressure is 13 bar or above.
 2. The method of claim 1,wherein additional water is added to the oily sludge waste materialbefore a flocculating agent is added to the waste material.
 3. Themethod of claim 2, wherein the water is added to the oily sludge in avolume/volume ratio of 2:1.
 4. The method of claim 2, wherein the amountof water added is such that the flocculating agent used in stage b) ofthe method is diluted to between 0.1 times and 50 times the volume ofthe oily sludge waste material.
 5. The method of claim 1, wherein, theflocculating agent is a synthetic polyelectrolyte of an acrylamidecopolymer.
 6. The method of claim 1, Wherein flocculation is promoted bymixing in a 0.6% solution of a cationic polyacrylamide flocculentemulsion polymer at a dose rate between 0.1 and 15 kg/tonnes of drysolids.
 7. The method of claim 1, wherein the filtering of the mixtureto extract the free water is carried out using a chamber press with asuitable filter clothing.
 8. The method of claim 1, wherein the secondpressure is maintained for 2 hours above 13 bar.
 9. The method of claim1, further comprising a pretreatment stage wherein the oily sludge wastematerial is screened to remove large debris from the sludge material.10. The method of claim 1, further comprising the washing of the oilysludge waste material in situ within the press with an oil dispersantsurfactant solution.